An assailant shot the driver, then climbed out a back window and waited for authorities, a Metro spokesman said.

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A bus passenger riding a bus with no other people aboard shot and killed the bus driver on Sunday, May 20, 2012 on Santa Monica Boulevard in West Hollywood.

Updated at 11:20 PM PST on Sunday, May 20, 2012

A Metro bus driver was shot and killed by a passenger in West Hollywood Sunday morning in an incident that has rocked the tight-knit community of transit riders and drivers throughout the region.

The driver, whose name has not yet been released, had just steered his bus onto Santa Monica Boulevard out of a layover area at La Cienega Boulevard when he was attacked, said Marc Littman, spokesman for the Metropolitan Transportation Authority, or Metro.

Gallery: Bus Driver Fatally Shot

The transit agency happened to have a supervisor stationed on Santa Monica to help set up a temporary road closure to make way for the Amgen Tour of California bicycle race, said another spokesman, Rick Jaeger. At a few minutes after 9 a.m., the supervisor saw the bus - which had been heading east - roll backward into a parked car, and ran over to see what was going on.

He found the wounded driver, who was quickly taken to Cedars Sinai Medical Center. He died there at about 9:30 a.m., Littman said.

Dramatic LA News Photos

It was the first time a Metro driver had been slain on the job, Littman said.

Preliminary reports indicated that the assailant used a shotgun, Littman said, and then climbed out of a rear window of the bus.

The gunman then waited for sheriff deputies to arrive, Littman said.

Los Angeles Sheriff's Lt. Dave Dolson said homicide detectives were trying to determine a motive and that it didn't appear the gunman had a relationship with the bus driver.

Dolson said there were no other passengers aboard.

"It's very very rare that something like this happens," Dolson said. "We'll know more in the next few days, I'm sure."

Metro CEO Art Leahy went to the scene, expressing condolences to the drivers' family and promising to make grief counselors available to the agency's 4,000 drivers, Littman said.

"It's an absolute tragic situation," said Jaeger. "We've lost one of our family members."